[llvm] [SSAUpdaterBulk] Add PHI simplification pass. (PR #132004)

[Valery Pykhtin via llvm-commits]

vpykhtin wrote:

The `EliminateDuplicatePHINodes` function has two main issues. First, it replaces later PHI nodes with earlier ones, leading to unexpected behavior where existing PHI nodes are replaced with new ones. Second, it tests all PHI nodes instead of focusing solely on the newly added ones, which would be more efficient.

https://github.com/llvm/llvm-project/pull/132004